This invention relates in general to parachute canopies, and more particularly to canopies that include stretch fabric. The invention is concerned with the reduction of the opening forces of a parachute where the opening occurs at high air speeds without increasing the opening time where the opening occurs at low air speeds.
In the manufacture of parachutes it has been known to provide canopies which will have a high permeability of porosity when subjected to opening shock and a lower porosity during descent.
U.S. Pat. No. 3,222,016 to Boone discloses a parachute using an unidirectional stretch fabric which achieves a variable porosity or permeability by stretching in response to the shock of opening. Boone uses a stretch fabric employing stretch yarn for the threads in one direction and a non-stretch yarn for the threads in the other direction. The purpose of this fabric is to provide a parachute having a greater porosity during opening shock and a lesser porosity during the descent which follows. The stretch fabric is used throughout the entire canopy of the parachute. One embodiment of the Boone canopy is formed from radial gores which are connected together by radial seams. Another embodiment of the Boone canopy employs circumferential panels of material arranged in concentric circles which are joined together by circumferential seams.
U.S. Pat. No. 3,655,152 to Bonn et al. also discloses use of a stretch fabric. In one embodiment of the Bonn patent, the radially inner portions of the canopy are of a unidirectional stretch fabric. The line of stretch of the fabric is at an angle of 45.degree. relative to radial lines of the canopy and thus also at an angle of 45.degree. relative to circumferential lines of the canopy. Bonn teaches that the radially outermost portion of the canopy may be formed from conventional non-stretch fabric.
In both patents, the stretch fabric serves the purpose of responding to shock during opening by stretching, thereby increasing porosity and thus reducing the shock during opening. The stretch fabric is about twice the weight of standard fabrics. Thus its use results in a bulkier, heavier parachute.
In the typical flat or near flat circular canopy design, the parachute initially forms a configuration much like a cylindrical tube or sausage-like arrangement extending up from the load being carried. During the first stage of opening, air travels through the tube to the top of the tube and forms a bubble at the top which then increases in size and extends down the canopy. Very roughly, the canopy takes the shape of a mushroom. The mushroom thereby formed grows up to a point at which the second stage of opening commences. During this second stage, the stem of the mushroom or remaining tubular portion of the parachute opens in a radial direction. The greatest forces tend to occur when this second stage begins. Thus it is important that the shock reduction be most effective at the start of this stage two of inflation.
It is desired to have this second stage proceed symmetrically and in a controlled fashion. But, when the portion which provides for the stretch is on the 45.degree. bias, distortion and non-symmetrical opening tends to occur. Where the stretch is in a radial direction, the cylinder or stem tends to elongate axially and this tends to counteract or disrupt the opening process. When the entire panel is stretch material, the greater mass due to the relatively heavy stretch material tends to create inertial effects which disturb and distort the opening process.
Accordingly, it is a purpose of this invention to provide a canopy design in which the opening shock reduction advantages of stretch fabric are obtained while maximizing the tendency of the parachute to open in a symmetrical and controlled fashion.
It is a further purpose of this invention to provide the most significant shock reduction effect at the point of greatest shock and thus, in particular, at the initial portion of the second stage of the opening process.
It is a further purpose of this invention to provide a technique for reducing opening shock which will permit a controlled opening that will not detract from the shock reduction.
The use of stretch fabric results in a substantial change in the configuration of the parachute during its opening stages and particularly in the stages where the air dynamic pressure is greatest. This change in configuration of the opening parachute can result in a distortion of the opening process which is undesirable. For example, it is important that a parachute open in as symmetrical a fashion as possible. The operation of the stretch fabric results in loss of some of the control of the opening configuration and can result in non-symmetrical opening. This loss of control can also result in a tendency to resist or delay the initiation of the opening process. It is very important that the initiation of the opening process occur immediately and this will occur only if the mouth of the parachute opens immediately upon the parachute obtaining its deployed or squidded condition.
Accordingly, it is a further purpose of this invention that the use of stretch fabric to reduce opening shock not delay the initiation of the opening process as well as that it minimize distortion of the opening process.
In determining the configuration and use of stretch fabric material as a portion of the parachute, there are a number of trade-offs which have to be taken into account. These include:
1. relative cost of stretch fabric material and ordinary fabric; PA1 2. relative bulkiness; PA1 3. configuration of parachute at point of maximum shock during opening; PA1 4. obtaining maximum use of stretch fabric material at point of maximum shock; PA1 5. minimizing the areas in which the stretch fabric material is tied down by seams and attachment to non-stretch material.
Accordingly, the overall purpose of this invention is to provide a design for the use of stretch fabric material that will optimize the effectiveness of the stretch fabric material at the point of maximum shock while minimizing the bulkiness and additional costs involved, yet providing an effective parachute for normal operating conditions other than at the point of maximum shock.